The separation of aromatic and nonaromatic hydrocarbons (generally referred to as dearomatization) from mixed hydrocarbon feeds has long been recognized as necessary and advantageous for a number of varied reasons. For example, when a BTX fraction (benzene, toluene and xylene) is the aromatic fraction it may be used as a raw material in the manufacture of petrochemicals, or as an additive for gasoline to increase its octane rating. Further, the nonaromatic fraction derived from these mixed feeds have varied uses as fuels, solvents and the like and, therefore, are also highly desirable. Such uses for the aromatic and nonaromatic fractions have resulted in the development of numerous dearomatization processes.
Of particular interest and difficulty is the separation of the complex components present in lube oils, wherein the removal of aromatic-type hydrocarbons is necessary to improve the viscosity index, thermal and oxidation stability, and color of the lube oils. The presence of aromatic-type hydrocarbons in lube oils affects the quality of these oils due to the low viscosity index, poor thermal and oxidation stability, high carbon residue, and poor color of such aromatic-type hydrocarbons. The aromatic-type hydrocarbons present in lube oils differ significantly from the BTX fraction found in light hydrocarbon mixtures used in the production of gasoline and, as a result, present vastly different separation problems.
Various processes have been suggested for the separation of the aromatic and nonaromatic hydrocarbons of a mixed feed wherein the aromatic is a BTX fraction. Typical of these processes is a process employing an extraction column for separation of a BTX fraction wherein a glycol solvent/water solution, BTX and a reflux stream is introduced to a two step distillation column. BTX is then distilled to remove water and entrained glycol. Similarly, a process has been suggested wherein two distillation columns are employed with the BTX fraction and water being distilled in the second column. In addition, a process using two distillation columns wherein the second column is employed to distill the BTX fraction and other components, has been suggested.
The aforementioned processes have not proven satisfactory when the separation of the aromatic hydrocarbon component has been other than a BTX fraction, particularly when the process was employed for the dearomatization of lubricating oils. Therefore, a number of processes have been proposed for the dearomatization of mixed hydrocarbon feeds containing various aromatic hydrocarbons. These processes have been directed, in large part, to the choice of the extraction solvent. For example, U.S. Pat. Nos. 2,400,732 and 2,402,799 disclose extraction/distillations employing a solvent containing primarily water as the extraction solvent. Numerous water based solvents have been suggested for the extraction of aromatic hydrocarbons from mixed hydrocarbon feeds but to date such extraction solvents have not proven to be satisfactory. These include water based extraction solvents such as: glycol/water wherein up to 50 percent glycol is added (U.S. Pat. No. 2,400,802); methanol/water (U.S. Pat. No. 3,985,644); water/non-oxygenated organic solvents (U.S. Pat. No. 2,298,791); water/amines (U.S. Pat. No. 2,401,852); and water/inorganic salts, acid or bases, or organic substances (U.S. Pat. No. 2,403,485). The problems associated with employing water-based extraction solvents, are well known in the prior art, e.g., use of extremely high pressures.
Various processes have been suggested to help overcome the problems associated with employing water based extraction solvents. For example, U.S. Pat. No. 1,783,203 discloses the use of dry alcohols (C.sub.1 -C.sub.3) for treating heavy petroleum oils. The problems relating to the flammability and toxicity of such alcohols are well known in the art. U.S. Pat. No. 1,908,018 discloses the use of certain ethylene glycol ethers, i.e., ethers of ethylene glycol and diethylene glycol, and their acyl derivatives in a process for refining mineral oils. The process involves separating the paraffinic and naphthenic portion by admixture of the oil feed with an ethylene glycol ether followed by cooling and agitation of the entire mixture to provide a more paraffinic upper layer and a more naphthenic lower layer. As is clear from the patent, the process is not directed to the separation of aromatic and non-aromatics. The solvent is then removed by vacuum distillation (see beginning at column 4, line 129 et seq.) The process does not employ solvent mixtures of ethylene glycol ethers with other solvents nor does the process employ glycol ethers above diethylene glycol ethers. Further the process is necessarily energy intensive owing to the use of distillation steps for the removal of the extraction solvent.
U.S. Pat. No. 2,337,732 discloses the use of ethanolamines for removing aromatics from a hydrocarbon distillate, comprising gasolines or light hydrocarbons (C.sub.1 -C.sub.5), by an extraction-distillation process. U.S. Pat. No. 2,295,612 discloses the use of low molecular weight polyhydric alcohols for separating aromatic mixtures to obtain resin-forming compounds. U.S. Pat. No. 2,129,283 discloses the use of a beta, beta'-dichloro diethyl ether and 2-30% propylene glycol as the solvent for extracting naphthenic impurities from lubricating oils at temperatures from 120.degree. F. to 200.degree. F.. U.S. Pat. No. 3,379,788 discloses the use of alkylene oxide adducts of phenyl glycidyl ether and U.S. Pat. No. 2,834,820 discloses the use of mixed alkylene oxide adducts of ethylene or propylene oxide as solvents in dearomatization processes.
To overcome the relatively low yields, purities and solvent recovery problems of the above processes several dearomatization processes have been suggested employing extraction and distillation. These include: solvent extraction-steam distillation processes (such as those disclosed in U.S. Pat. Nos. 3,417,033; 3,714,034; 3,779,904; 3,788,980, 3,755,154 and 3,966,589); processes employing multiple extraction zones and azeotropic distillation (e.g. U.S. Pat. No. 3,789,077); processes employing distillation and stripping columns (U.S. Pat. Nos. 4,048,062 and 4,177,137); and multiple distillation processes (e.g. U.S. Pat. No. 3,461,066). Unfortunately these processes employ costly distillation of the solvent. Further, high capital and energy costs are generally associated with employing such processes. Therefore, alternative processes have been sought whereby these problems may be minimized.
U.S. Pat. No. 3,431,199 discloses a method of separating aromatic hydrocarbons from a mixed hydrocarbon feed by use of solvents comprising diethylene glycol, dipropylene glycol, sulfolane and mixtures thereof. The process is directed to the separation of light aromatics by extraction at temperatures preferably between 80.degree. and 130.degree. C. and employs azeotropic distillation with acetone to effect separation of the aromatic hydrocarbons. The process preferably employs solvent with 2% to 8% by weight water.
U.S. Pat. No. 3,551,327 discloses an extraction distillation process which employs a sulfolane-type solvent.
U.S. Pat. No. 3,985,644 discloses a method of separating naphtha into aromatic and paraffin-rich fractions with a methanol-water mixtures. The solvent is separated from the aromatic-rich phase by lowering the temperature of the mixture. As indicated therein, the solvent comprises methanol/water mixtures. These are highly toxic and flammable mixtures.
U.S. Pat. No. 4,086,159 discloses a method for separating aromatic hydrocarbons from mixed hydrocarbon feeds by use of an ethoxylate alkane polyol solvent in an extraction-distillation process. The ethoxylated alkane polyol solvents high boiling point provides for the recovery of high boiling aromatics such as ethylbenzene and polysubstituted benzenes. The process necessarily requires sizable quantities of energy to carry out the energy intensive distillation steps.
U.S. Pat. No. 4,179,362 discloses a method for separating aromatic-containing petroleum fractions into aromatic-rich and non-aromatic hydrocarbon streams by use of a methanol/water extraction solvent (having at least 10 volume percent water in the extraction solvent) in an extraction zone at a temperature of about 150.degree.-450.degree. F. The extraction employs water in the extraction step to reduce hydrocarbon solubility in the aromatic-rich extract. The extraction step is followed by further additions of water (distilled water) to the aromatic-rich extract such that the water/methanol solvent contains at least 80% water, by volume. The water and methanol must then be removed by flash distillation, an energy intensive process, or by some other process such as using super critical CO.sub.2 as an extraction solvent. The use of methanol/water solvents for treating higher distillates tends to require higher process pressures and suffers from the safety constraints associated with methanol/water solvents, e.g., high flammability and high toxicity.
The above processes show the intense interest in developing a dearomatization process which lowers the cost of those processes heretofore used commercially. U.S. Pat. No. 3,985,644 mentions one such method for achieving this goal, i.e., by reducing the use of energy-intensive steps, e.g., distillation.
The dearomatization of lube oils is of particular interest. Dearomatized lubricating oils are, generally speaking, naphthenic-and or paraffinic-type viscous materials having a low rate of viscosity change with change in temperature, i.e., relatively high viscosity index, a high degree of thermal and oxidation stability, low carbon-forming tendency, good color, and high flash points. Lubricating oil feedstocks are generally recovered as distillates or bottoms from the vacuum distillation of crude oils. A crude lube oil fraction contains many different chemical components, e.g., paraffins, naphthenes, aromatics, and the like. In order to obtain refined lubricating oils of relatively good quality and high viscosity index, the practice has been to remove components, such as aromatic and polyaromatic compounds, which tend to lower the viscosity index of the lube oil. The removal of these aromatic components has heretofore been carried out by processes as above-described and processes such as disclosed in U.S. Pat. Nos. 2,079,885; 2,342,205; 3,600,302; 2,773,005; 3,291,728; 3,788,980; and 3,883,420.
U.S. Pat. No. 2,079,885 discloses a process for refining hydrocarbon oils containing aromatic and non-aromatic components by counter current extraction at elevated temperatures with selective solvents such as furfural or phenol, cooling the aromatic-rich extract and oiling out the raffinate and recycling the oiled out raffinate. Unfortunately such a process results in some raffinate losses in the aromatic-rich extract.
U.S. Pat. No. 2,342,205 discloses a solvent recovery scheme wherein aliphatic and aromatic hydrocarbons are washed with water and then distilled.
U.S. Pat. No. 3,600,302 discloses a method of upgrading petroleum distillate fractions by extraction with a solvent comprising an aromatic organic compound having a 6 membered ring containing at least one polar functional group, e.g., phenol, and a lower glycol ether such as ethylene glycol monomethylether or diethylene glycol monomethyl ether. The process employs conventional distillation means to separate the solvent from the aromatic and non-aromatic phases and employs the lower glcol ether to decrease the capacity of the phenol solvent owing to the extremely high capacity of phenol at the high extraction temperatures (see col. 3, line 25 et seq).
U.S. Pat. No. 2,773,005 discloses a process wherein light lubricating oils are extracted by use of phenol and water. The phenol is recovered from a second extract fraction wherein said extract fraction contains aromatic-type hydrocarbons and phenol (extraction solvent). Thus, the process requires regeneration of the extraction solvent by means of additional separation processes in that the "second extract fraction" contains phenol (a relatively toxic compound) and aromatic-type compounds.
U.S. Pat. No. 3,291,728 discloses a process wherein a raffinate and extract fraction from an extraction process are washed with 25 percent to 50 percent, by volume, water.
U.S. Pat. No. 3,788,980 discloses a process for the recovery of aromatic hydrocarbons wherein a feedstock is contacted with a mixture of water and a solvent. The mixture containing aromatics is introduced to a distillation zone maintained at the boiling point of the mixture of aromatics with steam being introduced at the bottom of the distillation zone. Thus, a distillation zone is necessarily employed to remove the aromatic-type compounds.
U.S. Pat. No. 3,883,420 discloses a process for removing aromatic hydrocarbons from an extract phase (containing aromatic and extraction solvent) by use of a mixture of steam and a lower molecular weight paraffinic hydrocarbon (solvent). The solvent is recovered by steam stripping or by extractive distillation followed by a solvent recovery column.
In copending application U.S. Ser. No. 164,039, filed June 30, 1980, commonly assigned, a solvent extraction-solvent decantation process is disclosed wherein solvent purification with mixed hydrocarbon feed or raffinate can be employed. The instant process eliminates these solvent purification steps.
In copending application U.S. Ser. No. 267,427, filed June 4, 1981, commonly assigned, there is disclosed a process for the separation of aromatic and nonaromatic containing feedstocks by use of a unique extraction-decantation process wherein the extraction solvent as preferably a low molecular weight polyalkylene glycol. The instant process provides for an improved process by use of improved mixed extraction solvents.
The process of this invention is to be distinguished from the prior art in that the instant process provides an extraction-separation process that is more economically advantageous, i.e., energy efficient, and overcomes problems inherent in many of the above-described processes.